Portable power is needed for a wide range of battlefield applications, from tactical radios to compact surveillance drones. For ruggedized applications requiring less than 500 Wh energy per charge at 24 V or less, lithium-based battery packs can deliver the energy needed. They have been used in applications ranging from vehicular subsystems (which are generally not the starting battery) to electronic elements of an infantryman’s Ground Soldier Ensemble. A battery pack typically includes the battery cells, some form of battery management functionality—such as protection circuitry and fuel gauging—and a digital bus for communicating with the equipment the pack is powering.

Battery chemistries are generally identified by their cathode material. For ruggedized battery packs, these include lithium cobalt oxide (LCO), lithium iron phosphate (LiFePO4), and nickel manganese cobalt (NMC) cells, although blended chemistries are emerging. LCO is used for applications needing high energy density such as hardened notebooks and PDAs and LiFePO4 is used where high discharge rates are required. To date, NMC is used mostly in electric vehicle applications.

Blended cathode materials allow manufacturers to tailor the performance of the cathode to the application. For example, by blending lithium cobalt oxide and lithium manganese oxide, Boston Power’s (www.boston-power.com) Swing® and Sonata® cells improve energy density, power, charge rate, calendar and cycle life, and operating-temperature range (Fig. 1). The Swing cells are designed for vehicular applications while the Sonata cells are suitable for notebook computer and portable industrial applications. The company offers cells, modules, and complete portable power systems.

The form factor for the cells inside a battery pack can be either cylindrical or prismatic. Cylindrical cells are usually described by their external dimensions (Fig. 2). For example, a “18650” cylindrical cell is 18 mm in diameter and 65 mm long. Its nominal voltage rating is 3 to 4 V and it will have a capacity around 2600 mAh. “Prismatic” is the terminology for a squarish cell that swells slightly when it is charged. Either kind of cell presents some manufactu-ring challenges to the battery pack maker: Cylinders don’t pack as densely as prismatics, but the pack manufacturer doesn’t have to accommodate the swelling.

Each approach has its advantages and disadvantages. For example, Boston Power claims a 40% to 50% cycle-life advantage for its manufacturing approach, which uses prismatic cells (rather than cylindrical) inside an aluminum (rather than steel or plastic) shell. That cycle life estimate is based on an analysis of the effects of frequent (i.e., once daily) mechanical stresses inside the pack caused by expansion and contraction of the cells as they charge and discharge. Both steel and aluminum have mechanical strength and thermal-conductivity advantages over plastic, although plastic has its own mechanical advantages in terms of resilience. Aluminum has the best thermal conduction characteristics which tend to give it an advantage in terms of a uniform thermal profile.

Discussions of packaging always lead to environmental considerations. Inevitably, battery packs for military field operation will include environmental requirements in the request for proposal (RFP), which will need to be demonstrated through qualification testing using the test procedures of MIL-STD 810. The most critical environmental requirements are likely to include one or more of the following: high and low temperature, humidity, sand and dust, explosive atmosphere, immersion, and shock and vibration. For instance, requirements often include operating temperatures from - 40° to + 80°C, vibration cycling with dwells at resonance points, and drop tests for shock. That’s in addition to requirements for resistance to blowing sand and dust and to salt and fresh water immersion. Beyond environmental testing, MIL-STD-461 EMI testing may also be required if charging regulators, fuel gauging, and other electronics are present inside the packs.

By far, the most challenging requirement for rechargeable Li-ion batteries is low temperature perfor-mance. While some RFPs specify battery discharge at -40°C, most rechargeable Li-ion cells are rated for discharge at -20° to +60°C and for charge at -20° to +45°C. There are Li-ion cells from manufacturers such as Saft specifically designed for cold temperatures, but they are large and come at a premium price. It may be possible to use newer types of Li-ion cells rated for low temperatures that are now being offered by companies like Boston Power. To validate claims of low-temperature tolerance, equipment makers may have to conduct their own system testing.

One option is to not use rechargeable cells at all. It is possible to realize the weight and energy density advantages of lithium by using primary (disposable) lithium cells rather than rechargeable cells. Some of these primary cells have operational temperature ranges as low as -40°C and even -80°C with low self-discharge characteristics. Common chemistries for primary cells are lithium manganese dioxide, lithium sulfur dioxide, and lithium thionyl chloride.

To retain the advantages of recharging, there are ways to deal with low operating temperatures from a system perspective. One would be to design the powered equipment so that on being turned on, it would discharge the batteries in short pulses into a dummy load before applying the operational load. This would warm the cells above ambient temperatures through self-heating. Since the pulsing would only be activated when a battery pack was embedded in the equipment and available for use, there would be only normal self-discharge while the battery pack was in storage.

Another possibility, if the battery pack were used for supplementary or backup power—say, in a vehicle that also had a lead-acid battery for starting and routine functions—would be to embed a heating element within the lithium battery pack and power it from the vehicle battery.

A third option might be to embed supercapacitors within the pack. Capacitors do not rely on chemical reactions and therefore operate down to much lower temperatures than chemical batteries. Supercaps, ideally with a buck regulator to stabilize their output, could provide initial energy until the battery cells reached maximum output and achieved self-heating.

The other side of thermal design concerns meeting the high-temperature environmental requirements of an RFP, which may request extending operating temperatures to + 80° C. Conventional Li-ion cells are characterized to discharge to +60°C and charge to +45°C, although there are niche Li-ion cells designed to discharge at up to +80°C and charge up to +60°C, but, like low-temperature cells, they typically come at a price premium.

Practical solutions to high temperature operation starts with thermal design. For example, in contrast to the low-temperature situation, one wants as little I2R loss as possible in this situation, which means low impedance in the high current path, as well as positioning power semiconductors away from the battery pack cells, and good conductive and convective cooling wherever heat will inevitably be generated.

Mechanical design for shock and vibration is outside the scope of this article, although one way lithium battery packs help in this regard is via their relatively low mass. Nonetheless, there are some things only a vibration table and a drop test can reveal. For some general rules of thumb, some considerations relative to mechanical construction are offered by battery-pack maker Micro Power:

• PC and ABS blends can accommodate substantial shock in the field, while enabling ultrasonic welding of a battery enclosure during final assembly.
• Glass-filled nylons offer greater stiffness and strength with low warpage.
• Insulation within the pack can protect critical components like circuit boards and cells. The selected insulation must have adequate thickness, be puncture resistant, withstand temperature changes without deformation, and resist compression set or creep.
• External to the plastic battery enclo-sure, an overmolded material (usually an elastomer) can provide external protection to the battery internals.

In the field, water (especially salt water) is the enemy’s best friend. Micro Power also offers advice about alternatives for sealing a battery pack’s plastic case. Ultrasonic welding, the company notes, is recommended, but gluing or screws and gaskets will also provide good results.